Morphometric analysis of the foramen magnum in sex estimation: An additional 3DCT study from Nepal on a larger sample

Abstract Background Estimation of sex of the skeletal remains plays a vital part in the identification of an individual. This study is focused on the morphometric measurements of the foramen magnum region and examining the accuracy of sexual dimorphism in the Nepalese population. Methods Measurements were obtained from 3D computed tomography (CT) scan of 261 Nepalese adult cranial bases with known age and sex. Length and breadth of the foramen magnum, length and breadth of right and left occipital condyles and maximum and minimum intercondylar distance were measured on the base of the skull CT images. Results The mean values for all parameters were higher in males than females except for the maximum intercondylar distance. Sex prediction done with discriminant function analysis could classify the skull with an overall accuracy of 70.5%–71%. Conclusions It can be concluded from the results that the morphometric study of the foramen magnum is less reliable for sex estimation in the Nepalese population.

the time. 7 Therefore, in forensic practice any method which is less than 80% accurate is considered less reliable. 7 As the base of the skull is made up of thick bony structure and is covered by a large mass of soft tissue, it is well protected during external insults like violence, air accident, arson, explosion, or mass disaster even though the craniofacial structures and other bones are damaged. 12,13 The foramen magnum is the largest foramen in the human skeleton and is one of the important landmarks of the basicranium. 14 This has attracted considerable interest among researchers who have undertaken several studies in different populations to determine sexual dimorphism using morphometry. 6 The use of calipers and calibrated paper strips were used previously to study the morphometry of the cranial base. 13 Recently, the 3D scans obtained after radiological imaging are gaining popularity. Morphometry is a less expensive method to determine sex with reasonable accuracy, especially in the context of fragmentary remains of victims of mass disasters and skeletonized bodies. 5 The use of univariate and multivariate functions by anthropologists has suggested that the sexual identity from the cranial samples can be accurately established from 65% to 88%. 13,[15][16][17] There are not many studies pertaining to morphometry and sexual dimorphism in the foramen magnum in the Nepalese population. A previous Nepalese study which was conducted with CT images of the skull bases of 50 male and 50 female subjects calculated the area of the foramen magnum and used discriminant function analysis and predicted sex with 75% accuracy. 18 This study was conducted to re-evaluate the morphometry of the foramen magnum along with the occipital condyles for sexual dimorphism by using discriminant function analysis in the Nepalese population by using CT scan images.

| METHODS
The present study was a descriptive cross-sectional study conducted upon the three-dimensional computed tomography (3DCT) images of the base of the skull of 261 Nepali individuals whose age and sex were known. All the CT images during the study period were from adults who presented to the Department of Radiology of Lumbini Medical College Teaching Hospital, Palpa for the CT of the head as indicated in the course of their management for diagnostic purposes during the period between February and September 2021.
The study was conducted on normal human skulls of patients (more than 20 years of age for both sexes and with matched ages) who underwent CT scans for various diagnostic medical or surgical indications. Only the high-quality reconstructed CT images were included and low-quality blurred images or those with artifacts and those which did not cover the entire area of the foramen magnum were excluded. Noncontrast CT head was done on SIEMENS SOMATOM scope 16 slice CT machine with 0.5 slice thickness and studied with OsiriX MD software. Scans of subjects where there was gross pathology or injury or any deformity in the skull base region were excluded.
For the purpose of the study, 3DCT images were analyzed and various parameters were measured on the console of the scanner on a centimeter scale ( Figure 1). Measurements less than 1 cm were expressed in millimeters which were converted to centimeters during data analysis. The parameters that were measured were 14 : • Foramen magnum length (FML): the distance between the basion and opisthion was taken as the antero-posterior length of the foramen magnum in the mid-sagittal plane  The normal distribution of the data was determined by normality plots.
The continuous variables were presented as mean ± standard deviation (SD). Male-female differences were tested using the independent t-test and the significance level of the test was defined at p < 0.05. Pearson's test was applied to examine the correlation between each foramen magnum dimension in both sexes. The foramen magnum dimensions were further analyzed using discriminate function (DF) analysis using the step-wise method for sex predictability. The construction of DF was based on the step-wise method. In a forward selection approach, the best combination of predictors that maximized the (predicted) group classification was identified based on F-tests.
A cross-validation approach was based only on those observations that were included in the analysis. An overall measure of accuracy was then presented based on the cross-validated analysis. Such analysis presents a form of sensitivity analysis and helps to gauge the robustness of the model performance obtained from the final selected model.
Due to the record-based nature of the study, informed consent was not required in the present study. The study was approved by the Institutional Review Committee of Lumbini Medical College (reference number: IRC-LMC/12-J/020). All the underlying data for the present study is available without restriction. 19

| RESULTS
A total of 261 CT scans from 136 males and 125 females were included in the study. The age ranged from 20 years to 85 years with a mean age of 45.12 ± 17.89 years.
The intra-and interobserver reliability was assessed in 25 samples with a two-way mixed model and absolute agreement, the ICC values of the foramen magnum and occipital condyle measurements were greater than 0.9 except for the maximum intercondylar distance (intraobserver ICC: 0.876, interobserver ICC: 0.853). All the data were found normally distributed on the Q-Q plot.
The mean values for all measured parameters were higher in males than females except for the maximum intercondylar distance in the present study. The sex differences in the mean values of all the measurements were analysed by the student t-test set at 95% confidence interval and significance set at p < 0.05 (Table 1). The results of the t-test showed no significative differences in WROC and MxICD.
The correlation between all the variables was examined using Pearson's test for both sexes as shown in Tables 2 and 3. In both the sexes a strong correlation was seen in FML/FMW (r = 0.676 in males; r = 0.618 in females). In males, the strongest correlation was observed in LROC/LLOC (r = 0.759) and in females, strongest correlation was observed in WROC/WLOC (r = 0.743). The strongest negative correlation was observed in FML/MxICD in both males and females.
The variables were then analysed for discriminant function using both univariate and multivariate methods ( canonical discriminant function was 0.424, eigen value 0.248, and the group centroid was 0.447 for males and −0.487 for females with a cut-off score of −0.02. When a step-wise method was adopted, the three variables FMW, LLOC, and MnICD showed higher classification accuracy (Table 5).
Cross-validation using a jack-knifed approach that is, for each observation, sex classification is obtained by using a function constructed from the remaining observations. This means that for each of the observations in the data set (N = 261), the classification (discriminant) function was derived using data from the remaining 260 observations. Based on the discrimination function analysis, the classification of the sex was estimated. Canonical discriminant function coefficients of the selected variables and sex prediction accuracy based upon these variables are presented in Table 6. The results show that the function provides an overall accuracy of 70.5% during sex estimation.

| DISCUSSION
The foramen magnum is chosen to identify the sex of partial or damaged skeleton in forensic caseworks. This is because it is a regular structure that is unlikely to undergo significant morphological changes. Its development can be used to describe the degree of sexual dimorphism exists within the dimensions of foramen magnum.
Compared to many other skeletal components, the foramen magnum matures relatively early in childhood and is unlikely to respond significantly to secondary sexual changes. The foramen magnum is not controlled by muscles; rather, it serves as a passageway for structures entering and exiting the cranial base region, particularly the medulla oblongata, which occupies the majority of the foraminal space. Because the nervous system is the most precocious of all body systems, it matures at a very young age and does not need to grow in size. The occipital bone fuses completely by the time a child is 5 to 7-year-old. 13 In 1967, Radinsky measured the foramen magnum length and breadth to calculate the area of the foramen magnum which he utilized to approximate the brain size in mammals. 20 In 1982, Teixeira measured the length and breadth of the foramen magnum of 40 exhumed skulls and stated that the area was larger in males than in females. 21 (Table 7). 1,2,4,6,[12][13][14][15][16]18,[24][25][26][27][28][29][31][32][33][34][35][36][37][38][39][40] The highest sex prediction was made by Holland who used a regression equation upon nine measurements from 100 crania and claimed the accuracy up to 90%. 38 Wescott et al. 39